Git integration

[rodionsteshenko]

Add native git support so users can diff staged changes, commits, or branches directly.

Desired behavior:

• diffuse --git to show staged changes
• diffuse --git HEAD~1 to diff against a specific commit
• diffuse --git branch1..branch2 to compare branches
• Show git status context (branch name, commit info)

Tech context: Rust backend can shell out to git or use the git2 crate. File list from multi-file support (#1) would pair well with this.